Modern power converters based on semiconductors are usually controlled by means of pulse width modulation (PWM). A corresponding control unit thereby generates a pulse pattern with modulated width in order to control the conversion of an injected electrical current (DC, AC) into the respective other, or to change characteristic parameters such as the voltage and the frequency. One of the most common applications of power converters is the control of variable speed electric motors, for example permanent magnet synchronous motors. The PWM frequency can be a constant value (for example 20 kHz) or adjustable during operation.
Thus, for example, in a 3-phase motor, by selectively switching each phase u, v, w by means of a PWM control from the DC voltage in the intermediate circuit an AC voltage is generated that drives the motor. The PWM control is provided with a measured current as the actual value (actual current) for controlling the pulse width, such that the PWM control can change the switching times and the pulse pattern of each phase, respectively, and thereby control and readjust, respectively, the amount, the frequency and the phase angle of the applied voltage.
The actual current can thereby be measured at discrete sampling times and from this PWM signals, i.e. the determined switch over and switching times, respectively, of the phases, can be determined. These are applied and used, respectively, output delayed by a sampling interval defined by two successive sampling times.
From DE 100 38 570 A1 a method for controlling the stator current of an electrical machine is known. In this case, currents are sampled at discrete points in time, and converted into a corresponding control output value, which is used with a delay of a PWM time step. Thereby, it is suggested to use a predictive actual current value for controlling the stator current. This allows for a compensation of the delay and to increase the bandwidth of the control loop. However, such a prediction is associated with disadvantages, in particular because of its sensitivity with respect to changes of engine parameters, by what ultimately the accuracy and stability of the control is affected.
It is an object of the present invention to enable an efficient and precise control of a current. In particular, the present invention is based on the object to provide a current control, for example, of an electrical machine connected to an inverter with high temporal dynamics.
These and further objects, which will become apparent from the following description, are achieved by the method according to claim 1 and by the device according to claim 11.